This invention relates to a method of making a retroreflective article having controlled divergence, and articles made by the method.
It is well known that retroreflective articles can be made from an array of microcube corner elements. Such an array of microcube corner elements can be made by ruling a master of “male” cube corners into a planar surface of a plate. This is taught generally by Stamm U.S. Pat. No. 3,712,706. It also is taught in detail in Pricone U.S. Pat. No. 4,478,769, assigned to the common assignee and incorporated herein by reference in its entirety.
U.S. Pat. No. 4,478,769 describes a well-known method of making triangular cube corner elements, in which the planar surface of a master plate is ruled with a diamond cutting tool that cuts a series of precise parallel vee-grooves. To rule equilateral triangular cube corners, three sets of parallel grooves in directions intersecting one another at angles of 60° are made; each groove also will have an included angle of substantially 70.53° disposed symmetrically, and will be ruled to a groove depth determined by the height of the cube corners desired. This method automatically results in an array of pairs of oppositely oriented equilateral triangular microcubes on the face of the master. To rule non-equilateral triangle cube corners the grooves within the parallel sets will contain angles other than 70.53°, and intersect at angles other than 60°, as disclosed, for example in Rowland U.S. Pat. No. 3,684,348. Methods for ruling non-triangle cube corners generally do not use three sets of parallel symmetrically disposed vee-grooves, but the faces of the cube corners are nevertheless formed from the walls of grooves, as disclosed, for example in Nelson U.S. Pat. No. 4,938,563. Methods for creating cube corner arrays other than by ruling a single plate have been disclosed in U.S. Pat. No. 6,015,214. The present invention applies to all microcube corner arrays, regardless of how they originated.
A master of “male” or “female” cube corner elements can be used to make a sequence of copies, of alternating gender, such as by electroforming. At any stage, copies can be assembled together, and the assemblies used to make further copies. After a series of assembly and copying stages, a single “mother” can be formed. The “mother” can be used to make production tools, such as by electroforming, which tools can be used to form microcube retroreflective elements on an expanse of plastic sheeting material such as by embossing, casting, compression molding or other methods known in the art.
Microcube corner retroreflective sheeting such as made by the method described above is used in highway safety applications such as highway signs and pavement markers. In such applications, the microcube corner elements reflect light from a vehicle's headlights back to the eyes of the driver of the vehicle. This is an inexact retroreflection in which the divergence angle, α, ranges between approximately 0° and more than 3°. The value of α operative in any given situation depends on the geometry of the vehicle and the driver and the distance from the vehicle to the retroreflective material. For example, the divergence angle α for a large truck's right headlight and its driver at a distance of about 40 meters from a road sign will be approximately 3°, while the divergence angle α for an automobile's left headlight and its driver at a distance of about 600 meters from a road sign will be approximately 0.05°.
Also associated with the divergence angle, α, is a rotation angle, ε, which is a measure of the direction of the divergence. The value of ε will be different for left and right headlights of a vehicle, and will also depend on the vehicle and driver geometry and the position of the road sign. For sheeting that will be mounted in random orientation on road signs, retroreflectance is required at every value of ε. The angles α and ε are defined in ASTM E808, Standard Practice for Describing Retroreflection, which document refers to divergence angle α as “observation angle”.
Ideally, microcube corner retroreflective sheeting used in road signs will produce a pattern of retroreflected light having sufficient intensity over a range of divergence angle values and rotation angle values. For example, even a non-urban retroreflective highway sign should retroreflect light through a divergence angle α of about 1°, which corresponds to the value of α from a large truck's right headlight back to its driver at a distance of about 120 meters from the road sign.
Improvements in the precision with which microcube corner elements can be ruled in a master plate and duplicated, particularly by embossing, have led to concerns that such microcube corner retroreflective sheeting may be adequately retroreflective over only a very narrow range of divergence angle, such as about 0.0–0.5 degrees. It would be preferred to provide an array of cube corners producing the entire desired range of divergence and within very short distances on the array so that a human observer of the article will see it as retroreflectively uniform.
Light that is retroreflected by micro-sized cube corner elements will experience a certain amount of diffraction because of the very small size of the microcubes. Such diffraction will result in retroreflection over broader ranges of both divergence angle and rotation angle. The particular ranges of α and ε will depend on the particular diffraction pattern of a given microcube, which will depend in turn upon the cube size, cube shape, the index of refraction of the cube material, and upon whether or not the cube faces have been metallized. Diffraction, however, is not a desirable method to enhance retroreflection through broader divergence and rotation angle, because the very small microcubes that achieve greater diffraction also cause a substantial quantity of light to be retroreflected with a divergence angle α of greater than about 3°, where the light is not useful to the vehicle driver. Diffraction also can result in idiosyncratic diffraction patterns that are unlikely to distribute the retroreflected light in a manner that will be useful to a vehicle's driver.
It is known in the art to produce divergent rectroreflectance by means of cube corner elements having intentional aberrations in respect of their dihedral angles deviating slightly from 90°. The classic paper “Study of Light Deviation Errors in Triple Mirrors and Tetrahedral Prisms,” J. Optical Soc. Amer., vol. 48, no. 7, pp. 496–499, July, 1958 by P. R. Yoder, Jr., describes the well-known spot patterns resulting from such aberrations.
U.S. Pat. No. 3,833,285 to Heenan, assigned to the common assignee and incorporated herein by reference in its entirety, teaches that having one dihedral angle of a macro-sized cube corner element greater than the other two results in extended observation angularity in microcubes, and specifically that the retroreflected light diverges in an elongated pattern.
When ruling an array of cube corners, dihedral angle errors may be produced either by causing the groove side angles to be slightly different from the design angles, as taught by Stamm U.S. Pat. No. 3,712,706, or by causing the angles of groove root crossings to be different from the design angles, or by combination of these methods.
U.S. Pat. No. 4,775,219 to Appeldom discloses retroreflective articles having tailored divergence profiles, wherein the cube corner elements are formed by three intersecting sets of parallel vee-grooves, and wherein at least one of the sets includes, in a repeating pattern, at least two groove side angles that differ from one another. Nelson U.S. Pat. No. 4,938,563 extended the method of U.S. Pat. No. 4,775,219 to non-repeating patterns of groove side angle differences.
U.S. Pat. No. 6,015,214 to Heenan et al., assigned to the common assignee, teaches methods of forming microcubes by ruling vee-grooves into the edges of a plurality of flat plates, and discloses that the tilt angle of a cutting tool with respect to the plate edges being ruled can be adjusted continuously as each groove is cut as a function of the distance traveled by the cutting tool across the plate edges.
Pending U.S. patent application Ser. No. 10/167,135, filed Jun. 11, 2002, claiming the benefit of Ser. No. 60/297,394, filed Jun. 11, 2001, discloses retroreflective articles and a method of making retroreflective articles having controlled broader divergence produced by ruling three intersecting sets of parallel V-shaped grooves in which ruling non-uniform deviations of the cube dihedral angles from exactly 90° are intentionally introduced by causing the cutting tool and the surface of the substrate to oscillate with respect to one another in a controlled manner during the ruling of at least one of the vee-grooves.
It is thus one object of the invention to provide an article comprising an array of retroreflective microcube corner elements having controlled broader divergence.
It is another object of the invention to provide a method for making such an article.